It is well known that DC motors, which require commutation to operate, are superior to AC or synchronous type motors in terms of torque characteristics, size, weight, and response. However, the commutators and brushes required in DC motors have the disadvantages that the brushes wear out, generate high-frequency noise, and add substantially to the manufacturing cost of the motor. Various circuits have been developed for operating a permanent magnet motor as a brushless DC motor, using an external sensor to sense the position of the rotor. The sensor is used as a commutator to sequentially energize the stator windings. Sensors, such as Hall effect devices and optical encoders, for example, have been used for providing commutation. It has also been recognized that the waveform of the induced or back EMF generated by the rotation of the permanent magnet field relative to the stator windings can be used to indicate the position of the rotor. The use of the back EMF has the advantage that no additional components or sensors are required. There is no alignment problem, which can be a critical problem in obtaining maximum torque and efficiency where a large number of commutation segments are involved. The disadvantage of such systems, however, is that the motor must be rotated before any back EMF signal can be generated. Thus such systems require some type of starting mechanism.
Known systems using back EMF have proved less responsive and lack the reliability of systems using external sensors. Examples of prior art systems of this type are found in U.S. Pat. Nos. 3,304,481 and 3,611,081. A system using resistors in series with the stator windings for generating a voltage in response to the back EMF is described in the publication "Waveform Sensing Closes the Loop in Step Motor Control", by J. R. Frus and B. C. Kuo, Products Engineering, February 1977, pages 47-49. These circuits, however, have not proved entirely satisfactory either because they require some adjustment in the circuit based on the speed to obtain maximum torque, or they are limited to only one winding being energized at a time in the stator.